In the field of two stroke engines, it is known to mix lubricating oil in mist form with the intake mixture in order to lubricate various sliding parts of the engine which are exposed to the mixture. Mixing lubricating oil with the fuel may be accomplished by mixing the lubricating oil with the liquid fuel which is then injected into the intake air or by spraying oil mist into the intake air separately from the fuel. For instance, JP2014-20314A discloses an arrangement where lubricating oil stored in an oil tank is fed to and injected into an intake passage by using a diaphragm oil pump.
However, the need for an oil pump for supplying lubricating oil inevitably increases the size and complexity of the engine so that the engine may not be commercially acceptable for use as a general purpose engine. Furthermore, when lubricating oil is injected into an intake passage in mist form, the lubricating oil is deposited on the wall surfaces of the intake passage (including the crankcase chamber and the scavenging passage), and stays thereon. When such deposited lubricating oil is sucked into the combustion chamber in any substantial amount, the resulting combustion of the lubricating oil may cause an increase in THC (total hydrocarbon content) in the emission, generation of white smoke and abnormal combustion. Therefore, when the lubricating oil is mixed with the intake in mist form, it is desirable to recover the lubricating oil deposited on the wall surfaces.